As a technique for stopping a fuel cell system, there has heretofore been proposed a method of purging flammable gas from the fuel gas passage of the fuel cell system by use of inert gas such as nitrogen. This purge method using inert gas, however, disadvantageously requires additional provision of a feeding system such as a nitrogen gas cylinder or Ar gas cylinder.
Various shutdown methods without use of inert purge gas have been proposed. A known method uses air for purging the fuel gas passage of a fuel cell system. This method will be outlined below.
As illustrated in FIG. 5, a fuel cell system 39 has, as chief components, a solid polymer electrolyte membrane type fuel cell 1 having an anode 1a and a cathode 1c; a fuel processor 2 having a reformer (not shown) for generating hydrogen-rich fuel gas by adding water to city gas or natural gas to reform it, which city gas or natural gas serves as a power generation material gas; a water feeder 3 for supplying water to the reformer of the fuel processor 2; a material gas feeder 6 for supplying the power generation material gas to the reformer of the fuel processor 2; a combustor 4 for combusting remaining fuel gas that has been discharged without being consumed in the anode 1a of the fuel cell 1; a blower 5 that serves as an oxidizing gas feeding device for supplying oxidizing gas (air) containing oxygen to the fuel cell 1 to discharge remaining gas outside from the fuel cell 1; and a purge air feeder 26 for supplying purge air for purge treatment of the inside of the fuel processor 2 when stopping the power generation of the fuel cell system 39.
In the fuel cell system 39, a reaction between the hydrogen-rich fuel gas supplied as the fuel gas to the anode 1a of the fuel cell 1 and air supplied as the oxygen-containing oxidizing gas to the cathode 1c of the fuel cell 1 is caused for power generation within the fuel cell 1, and at shutdown of the fuel cell system 39, the fuel gas passage is finally purged by air. A controller 21 properly controls the blower 5, the material gas feeder 6, the water feeder 3, the air feeder 26 and others to perform the above power generation and shutdown operation.
More concretely, when stopping the power generation of the fuel cell, hydrogen-containing fuel gas remaining within the fuel gas passage is removed by vapor which has been generated by supplying water from the water feeder 3 to the reformer of the fuel processor 2. Then, air from the purge air feeder 26 is allowed to flow into the fuel gas passage, thereby finally performing air purge (see Japanese Patent Document 1).
Compared to the conventional purge treatment process in which when stopping the power generation of the fuel cell system, nitrogen gas is allowed to flow into the fuel processor 2 and the fuel cell 1 so that remaining gas (fuel gas etc.) within these members 2, 1 is guided to the combustor 4 and undergoes treatment within the combustor 4, the above fuel cell system 39 can obviate the need for a storage for storing nitrogen gas so that it can attain cost reduction. The above technique has another advantage that air is supplied to the inside of the fuel cell after the removal of hydrogen gas from the fuel cell by use of vapor, thereby preventing the corrosion of the passages due to water droplets generated from vapor.
Apart from the above shutdown method, there is known another technique (see Patent Document 2) according to which when stopping power generation, air leakage into the anode of the fuel cell is prevented by introducing fuel gas (e.g., hydrogen-rich fuel gas) or power generation material gas (e.g., city gas or natural gas) into the anode and confining it therein, so that the durability of the fuel cell is maintained.    Patent Document 1: International Publication No. WO01/97312    Patent Document 2: Japanese Laid-Open Patent Application Publication No. 2003-282114